1. Field of the Invention
The invention relates to charging rollers for use in xerographic reproduction machines.
2. Description of the Background Art
In a xerographic copy machine electric charge is applied to a photoreceptor drum (PRD). An image to be copied is scanned with a strong light source and then reflected to the photoreceptor drum. The light dissipates the charge on the PRD where there is no reflected image. The reflected image, which is now in the form of patterns of charges on the PRD, attracts particles of toner. The toner is typically a carbon black pigment with a thermoplastic binder. The particles of toner are transferred to the substrate (paper) and bonded to it using heat and pressure to form the completed copy. In another system, the charge may be first transferred to the substrate so that the toner is attracted to the substrate rather than to the PRD.
Depending on the technology of the copying system, both the electric charge and the toner can be delivered to the proper location by different means. Electric charge may be applied to the PRD by a corona charging wire or by a charge transfer roller, also more generally referred to as a charging roller.
If the charge is applied with a roller, the charging, discharging, and capacitance characteristics of the roller surface are important factors to the operation of the system. The charge transfer roller surface is charged to the proper voltage. Charge is transferred to the PRD. The charge transfer roller surface is then recharged for the next cycle. Prior to recharging, it may be discharged to produce a uniform surface and starting point for the next charging cycle.
Charge transfer rollers typically are coated or covered with a layer of semiconductive material. Coating materials can include rubber, thermoplastic, or thermoset compounds containing carbon black or other low resistance additives, and anodized aluminum with special sealers to give the proper electrical properties.
The surface layer of the charge transfer roller has both volume resistance properties and capacitance properties. For charging and discharging the charge transfer roller surface, the surface layer functions electrically as an RC series circuit, a resistor and capacitor in series. The layer therefore has a time constant, which is a function of the product of the resistance and capacitance (R*C). For a roller surface layer, this may be expressed in seconds per unit area (e.g. microseconds per square millimeter or seconds per square inch).
The time constant determines the rate at which the surface layer may be charged and discharged independent of the applied voltage (unless the resistance or capacitance are voltage dependant). Series RC circuits charge and discharge according to a certain well known exponential function of time. When time t=RC, the charge has increased to within 1/e of its final value, where the numerical value of e is 2.718. It takes one time constant to charge the capacitor in the RC circuit to 63.2% of the applied voltage and three time constants to charge to about 95%. The time constant of the surface layer determines the maximum rate (copies per minute) at which the charge transfer roller may effectively function in the system.
In addition to the time constant of the surface layer, the surface layer must also have sufficient dielectric strength to resist the applied voltage without arcing through the layer to the core of the charge transfer roller (which is either grounded or held at a fixed bias voltage).
If toner is applied to, or comes in contact with, the charge transfer roller, there may be a doctor blade (or other cleaning mechanism) that would cause abrasion and wear of the charge transfer roller surface, thereby changing its properties. Thus, a very abrasion resistant charge transfer roller surface coating is highly advantageous for extending the service life of the charge transfer roller.
Since the charge transfer roller must transfer a uniform surface charge, there may be tight dimensional tolerances on the diameter, runout, and taper of the roller surface, as well as a specified and uniform surface roughness.
One of the common materials used for the roller surface layer is a specially sealed, anodized aluminum. This material has the following disadvantages:
1) The thickness of a high quality electrical grade anodized surface layer is limited to about 50 to 75 microns prior to any finishing operations, thereby limiting its dielectric strength.
2) Anodized layers are extremely porous and subject to dielectric failure from pinholes in the material. Even though the layer is primarily aluminum oxide, the porosity limits the compressive strength of the coating and its abrasion resistance.
3) In order for a high quality anodized surface layer to be formed, a high quality aluminum alloy must be used for the core body of the charge transfer roller. Also, the core body must be finished to tight dimensional tolerances (probably by diamond tooling) before applying the anodization process to produce a layer of uniform dimensions and electrical properties. Even so, the anodized coating thickness and properties may vary due to non-uniformities in the anodization bath and system.
4) The time constant of the layer may vary by plus or minus one order of magnitude (1/10 to 10X).
Rubber and thermoset surface layers have the following disadvantages:
1) Control of electrical properties through the use of additives is very difficult. The electrical resistance of the layer can easily vary by a factor of 100. Large variations within a single roller are also possible.
2) The abrasion resistance is low (especially rubber) compared to anodized aluminum.
3) Organic polymers age due to exposure to heat, chemicals, and oxygen. This changes and deteriorates their physical and electrical properties over time.
4) The electrical additives can themselves evaporate, leach out, bleed out or change (such as the breakdown of carbon black).
5) The process of applying the material to the metal core (molding, extrusion, etc.) can produce porosities and non-uniformities in the coating that affect its performance.
The present invention is intended to overcome the limitations of the prior art.